1. Field of the Invention
The present invention relates generally to a mobile communication system, and in particular, to a method for efficiently providing a multicast service in a hierarchical cell structure.
2. Description of the Related Art
In a mobile communication system, various wireless accesses to, for example, a megacell, a macrocell, a microcell and a picocell can be made according to a service range and a data rate defined in International Telecommunication Union-Telecommunication Standardization Sector (ITU-T) Q.1711, and service coverage is different according to the type of the cells. Also, the service coverage and a data rate can be different to some extent according to a class of a mobile communication service a service method for each of the cells will be described below by way of example.
The megacell provides a wideband service that covers a wide area using a satellite communication network, and its cell radius is less than 100 to 500 Km. The megacell can simultaneously provide a data service and a voice service. The macrocell has a service radius less than 35 Km, and is mainly used in the suburbs and a semi-flat area to expand its coverage in a low-traffic area in a mobile communication system. The macrocell can be introduced regardless of the frequencies between a base station for cell expansion and a base station installed as a repeater.
Meanwhile, the microcell has coverage of about 0.5 to 1 Km, and in the microcell, a visible-range radio path where a mobile station can see a base station's antenna becomes a main transmission path. In addition, the picocell services a building-crowded area and an underground area with a radius of less than 50 m in the downtown area by installing an in-building system or a small repeater. The in-building system provides an in-building service by connecting a European Transmission Rate 1 (E1) line from a mobile switching center to an in-building Base station Transceiver System (BTS), i.e., a pico-BTS, and installing an optical cable and an antenna in the building, and the small repeater receives a ground wave and services a basement of the building using an omni-antenna or a patch antenna at a maximum data rate of 2 Mbps.
In a mobile communication system, a service area and an application area of a cell are different according to the type of service provided, and an arrival distance and subscriber capacity within a cell area are changed according to a position of a base station's antenna, height of the antenna, a beam type of the antenna, an antenna slope, and transmission power of the antenna.
Providing different services according to a scale and data rate of a cell in the same area, as mentioned above, is called a “hierarchical cell structure (HCS).” For example, when a service for a microcell and a service for a macrocell are simultaneously provided in a particular area, it can be considered that the area is forming HCS.
Generally, in the HCS system, the macrocell manages a fast-moving user and a low-speed data transmission service, while the microcell manages a slow-moving user and a high-speed data transmission service. More specifically, since the macrocell has a large cell radius but has a limited service capacity, it is preferable for the macrocell to service a user requiring a low-speed data transmission service. However, when a fast-moving user is serviced in the microcell, service efficiency may be reduced due to frequent handovers (also known as handoff). Therefore, it is preferable that the fast-moving user is serviced in the macrocell rather than the microcell.
In some cases, a method of providing a service can be changed according to a load of the macrocell or the microcell. That is, there is a proposed method for providing a service to a fast-moving user in the macrocell when a particular microcell has a very heavy load.
Meanwhile, regarding a scheme for providing a multicast service in the HCS, there is a possible method for providing the multicast service in a microcell when the number of microcells providing a particular multicast service is very small, and for providing the multicast service in a macrocell when the number of the microcells is large.
However, for the multicast service, when a high-speed data transmission service is provided exclusively by a microcell, quality of service (QoS) may be decreased due to an increase in the number of users receiving a particular multicast service or to frequent handovers. In contrast, when the multicast service is provided exclusively by the macrocell, an increase in a data rate of the multicast service does not affect a load of the macrocell.
In addition, there is provided a scheme for dynamically selecting a layer of a multicast service. However, since handovers may frequently occur between layers during the service, the macrocell may experience considerable capacity reduction due to data being transmitted at high speed.